1. Field
The present invention relates generally to data communication, and more specifically to a novel and improved method and apparatus for a communication system employing multiple handoff criteria.
2. Background
Wireless communication systems are widely deployed to provide various types of communication such as voice, data, and so on. These systems may be based on code division multiple access (CDMA), time division multiple access (TDMA), or some other modulation techniques. A CDMA system provides certain advantages over other types of systems, including increased system capacity.
A CDMA system may be designed to support one or more CDMA standards such as (1) the “TIA/EIA-95-B Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System” (the IS-95 standard), (2) the “TIA/EIA-98-C Recommended Minimum Standard for Dual-Mode Wideband Spread Spectrum Cellular Mobile Station” (the IS-98 standard), (3) the standard offered by a consortium named “3rd Generation Partnership Project” (3GPP) and embodied in a set of documents including Document Nos. 3G TS 25.211, 3G TS 25.212, 3G TS 25.213, and 3G TS 25.214 (the W-CDMA standard), (4) the standard offered by a consortium named “3rd Generation Partnership Project 2” (3GPP2) and embodied in a set of documents including “TR-45.5 Physical Layer Standard for cdma2000 Spread Spectrum Systems,” the “C.S0005-A Upper Layer (Layer 3) Signaling Standard for cdma2000 Spread Spectrum Systems,” and the “C.S0024 cdma2000 High Rate Packet Data Air Interface Specification” (the cdma2000 standard), and (5) some other standards. These named standards are incorporated herein by reference. A system that implements the High Rate Packet Data specification of the cdma2000 standard is referred to herein as a high data rate (HDR) system. The HDR system is documented in TIA/EIA-IS-856, “CDMA2000 High Rate Packet Data Air Interface Specification”, and incorporated herein by reference. Proposed wireless systems also provide a combination of HDR and low data rate services (such as voice and fax services) using a single air interface.
Given the growing demand for wireless data applications, the need for very efficient wireless data communication systems has become increasingly important. There are many differences between voice and data services. One significant difference between voice services and data services is the fact that the former imposes stringent and fixed delay requirements. Typically, the overall one-way delay of speech frames must be less than 100 msec. In contrast, the data delay can be a variable parameter used to optimize the efficiency of the data communication system. Specifically, more efficient error correcting coding techniques which require significantly larger delays than those that can be tolerated by voice services can be utilized. An exemplary efficient coding scheme for data is disclosed in U.S. Pat. No. 5,933,462, entitled “SOFT DECISION OUTPUT DECODER FOR DECODING CONVOLUTIONALLY ENCODED CODEWORDS”, issued Aug. 3, 1999, assigned to the assignee of the present invention and incorporated by reference herein.
A second significant difference between voice services and data services is that the former requires a fixed and common grade of service (GOS) for all users. Typically, for digital systems providing voice services, this translates into a fixed and equal transmission rate for all users and a maximum tolerable value for the error rates of the speech frames. In contrast, for data services, the GOS can be different from user to user and can be a parameter optimized to increase the overall efficiency of the data communication system. The GOS of a data communication system for a user is typically defined as the total delay incurred in the transfer of a predetermined amount of data, hereinafter referred to as a data packet.
A third significant difference between voice services and data services is that the former requires a reliable communication link which, in the exemplary CDMA communication system, is provided by soft handoff. Soft handoff results in redundant transmissions from two or more base stations to improve reliability. A variety of soft handoff techniques are known in the art, and specific techniques are detailed below. Because data is transmitted at high rates, the effects of soft handoff on system capacity is severe. In addition, packets received in error can be retransmitted. For data services, the transmit power used to support soft handoff can often be more efficiently used for transmitting additional data. An exemplary system employing soft handoff is disclosed in U.S. Pat. No. 5,056,109, entitled Method and Apparatus for Controlling Transmission Power in a CDMA Cellular Mobile Telephone System” issued Oct. 8, 1991, assigned to the assignee of the present invention and incorporated by reference herein.
The most important parameters which measure the quality and effectiveness of a data communication system are the transmission delay required to transfer a data packet and the average throughput rate of the system. Transmission delay does not have the same impact in data communication as it does for voice communication, but it is an important metric for measuring the quality of the data communication system. The average throughput rate is a measure of the efficiency of the data transmission capability of the communication system. Factors involved in measuring the quality and effectiveness of data service to a particular user include maximum or minimum throughput rates to the user while that user has access to the communication channel, as well as the frequency of access granted to that user. These factors are relevant in providing a certain GOS to the user.
In CDMA systems, the signal-to-noise-and-interference ratio (C/I) of any given user is a function of the location of the user within the coverage area. The C/I that any given user's mobile station achieves on a particular link from the base station determines the information rate that can be supported by that link. Given a specific modulation and error correction method used for the transmission, a given level of performance is achieved at a corresponding level of C/I. The C/I achieved by any given user is a function of the path loss, which for terrestrial cellular systems increases as r3 to r5, where r is the distance to the radiating source. Furthermore, the path loss is subject to random variations due to man-made or natural obstructions within the path of the radio wave. The optimal performance occurs when the mobile station is served by the best base station, defined in terms of the largest C/I value received.
A system similar to an HDR system is disclosed in U.S. Pat. No. 6,574,211 (hereinafter the '211 patent), entitled “METHOD AND APPARATUS FOR HIGHER RATE PACKET DATA TRANSMISSION”, issued Jun. 3, 2003, assigned to the assignee of the present invention and incorporated by reference herein. In systems such as these, the characteristics, described above, that differentiate data communications from voice communications are exploited to provide efficient high speed wireless data transfer. These systems are summarized as follows.
Each mobile station communicates with one or more base stations and monitors the control channels for the duration of the communication with the base stations. The control channels can be used by the base stations to transmit small amounts of data, paging messages addressed to a specific mobile station, and broadcast messages to all mobile stations. The paging message informs the mobile station that the base station has data to transmit to the mobile station.
Upon receipt of the paging messages from one or more base stations, the mobile station measures the signal-to-noise-and-interference ratio (C/I) of the forward link signals (e.g. the forward link pilot signals) and selects the best base station using C/I measurements. C/I can be measured at the mobile station using a variety of known techniques, such as measuring a pilot signal, a broadcast channel, or any known signal from a base station. At each time slot, the mobile station transmits to the selected base station on a dedicated data request (DRC) channel a request for transmission at the highest data rate which the measured C/I can reliably support. The selected base station transmits data, in data packets, at a data rate not exceeding the data rate requested by the mobile station on the DRC channel. By transmitting from the best base station at every time slot, improved throughput and transmission delay are achieved.
The selected base station transmits at the peak transmit power for the duration of one or more time slots to a mobile station at the data rate requested by the mobile station. In CDMA voice communication systems, such as IS-95, the base stations operate at a predetermined back-off (e.g. 3 dB) from the available transmit power to account for variations in usage. Thus, the average transmit power is half of the total available power. However, in data systems such as HDR and that described in the '211 patent, data transmissions are scheduled in advance, it is not necessary to back-off from the available peak transmit power.
The ability to transmit from different base stations from time slot to time slot allows such data communication systems to quickly adapt to changes in the operating environment. In addition, the ability to transmit a data packet over non-contiguous time slots is possible through the use of sequence numbers to identify the data units within a data packet. Accomplishing this can introduce complexity, however, since often base stations independently select rates, and other base stations often can't read the DRC, so other base stations can be unaware of how much data is transmitted in any given slot. Often this task must be carried out via ACKs and NACKs (described in further detail below). In addition to sending a sequence number (or packet ID), an ID plus a rate may need to be delivered. On the backhaul, a base station can send a message including the packet ID and the quantity of data delivered in that packet.
Such systems increase flexibility by forwarding the data packets addressed to a specific mobile station from a central controller to all base stations which are members of the active set of the mobile station. The active set is a set of base stations selected from those neighboring a mobile station, typically selected based on the quality of the signals received at the mobile station. In these systems, data transmission can occur from any base station in the active set of the mobile station at each time slot. Due to the complex requirements of maintaining data queues in many base stations, and the associated backhaul traffic, the frequency with which mobile stations can select new base stations for transmission may be limited to minimize these effects. For example, a mobile may be required to stay with a particular base station for a given number of time slots, a certain time duration, or for a particular amount of data transmitted.
By queuing forward link traffic data to a variety of base stations, typically those contained in a mobile station's active set, one or more of these base stations can transmit data to the mobile station with minimal processing delay. The overall capacity of the system can be increased by reducing the forward link transmit power to any particular mobile station. Thus, system capacity is optimized by reducing transmit power directed to a mobile station by those base stations in the active set, as well as directing only a subset of the active set to transmit to that mobile station, such that the minimum C/I required is received at the mobile station. The technique of queuing data to a variety of base stations, and then directing a subset of them to transmit based on the specific mobile station's environment is known as armed handoff. A description of armed handoff is disclosed in U.S. Pat. No. 6,307,849, entitled “METHOD AND APPARATUS FOR CHANGING FORWARD TRAFFIC CHANNEL POWER ALLOCATION”, issued Oct. 23, 2001, assigned to the assignee of the present invention and incorporated by reference herein.
As described above, data systems such as HDR and that described in the '211 patent maximize throughput by scheduling the entire forward link channel for transmission to a specific user. Armed handoff is utilized with the limitation that only a single base station from the active set transmits during each time slot. Therefore, the power that would be required to allow soft handoff is redirected for use in data transmission. While this technique may optimize overall system capacity, issues can arise if providing a minimum GOS for particular users is desired. For example, in a deployed system, there may be geographical locations where the maximum C/I received by a mobile station from any one base station allows only a relatively low data transfer rate to the mobile station. This situation may arise due to the physical locations of the base stations in conjunction with various natural or man-made obstructions in the transmission paths. It may be that the desired GOS for that user can be attained by allocating additional medium access time for that user, i.e., assigning it additional time slots. In this case, servicing that mobile station may significantly reduce the capacity of that portion of the network. In the extreme case, however, even continuously allocating the entire forward link from any one base station may not provide enough throughput required to provide the desired GOS for that user. In these cases, allowing more than one base station to transmit forward link data simultaneously (i.e. soft handoff) may provide the additional C/I required to achieve the desired GOS to the particular mobile station while increasing overall capacity available to other users.
A retransmission mechanism can be employed for data units received in error. In systems such as HDR and that described in the '211 patent, each data packet comprises a predetermined number of data units, with each data unit identified by a sequence number. Upon incorrect reception of one or more data units, the mobile station sends a negative acknowledgment (NACK) on the reverse link data channel indicating the sequence numbers of the missing data units for retransmission from the base station. The base station receives the NACK message and retransmits the data units received in error.
Note that the benefits of allowing different base stations to transmit portions of the forward link data during various time slots as well as the retransmission mechanism require some coordination to maintain the queues in the various base stations contained in the active set. As an example, suppose a first base station is selected to transmit the first segment of data from its queue to a mobile station. After transmitting this data, the first base station knows that the second segment of data in its queue should be transmitted next, barring a retransmission request for the first segment. Suppose further that, after the first segment of data is transmitted from the first base station, the mobile environment changes such that a second base station is selected for a subsequent transmission. There must be coordination such that the second base station knows to transmit the second segment as opposed to the first. This coordination introduces some complexity as well as network traffic on the backhaul network (that is, the network between the base station controller and the various base stations). Backhaul network traffic is increased as more base stations are introduced to an active set, since coordination messaging increases proportionally, as well as delivering the forward link traffic data for storage in the additional base station's queue. Furthermore, it is clear that as data systems achieve higher data rates, the required data transfer on the backhaul network can increase dramatically.
Base station candidates for a mobile station's active set can be selected according to a variety of techniques. One such technique is disclosed in U.S. Pat. No. 6,151,502 (hereinafter the '502 patent), entitled “METHOD AND APPARATUS FOR PERFORMING SOFT HANDOFF IN A WIRELESS COMMUNICATION SYSTEM”, assigned to the assignee of the present invention and incorporated by reference herein. Using this technique, a base station can be added to the active set of the mobile station if the received pilot signal is above a predetermined add threshold and dropped from the active set if the pilot signal is below a predetermined drop threshold. Alternatively, the base station can be added to the active set if the additional energy of the base station (e.g. as measured by the pilot signal) and the energy of the base stations already in the active set exceeds a predetermined threshold. Using this alternative technique, a base station whose transmitted energy comprises an insubstantial amount of the total received energy at the mobile station is not added to the active set.
It is well known in the art that soft handoff techniques and active set selection techniques apply to sectors within a base station as well as to base stations themselves. However, the issues regarding backhaul network congestion described previously do not apply equally to sectors within a base station. This is because multiple sectors within a base station can share a common queue of forward link data. Therefore, when a sector of a base station is added to an active set already containing another sector of that base station, additional forward link data does not need to be delivered on the backhaul network. Furthermore, complexity arising from coordination of transmitted data from that base station's queue among that base station's sectors is less than that required for coordination of transmitted data between base stations.
As described above, wireless data users require high speed data transmission, and may have certain minimum grade of service (GOS) requirements. There is therefore a need in the art for communication systems which can provide maximum quality service for all users while efficiently managing backhaul network traffic and optimizing overall system capacity.